Advances in non-volatile memory technology include resistance-based memory technologies, such as Magnetic Random Access Memory (MRAM). MRAM technology is an emerging non-volatile memory technology that employs ferromagnetic-based Magnetic Tunnel Junctions (MTJs) as the basic memory elements.
A commonly used array architecture for MRAMs is the one-transistor, one-MTJ (1T1MTJ) architecture. As the name suggests, each bit cell in this architecture consists of an MTJ connected in series with an NMOS access transistor. In order to leverage the increased density and area reduction advantages associated with scaling down MOS technologies, it is desirable to use core transistors within the MRAM bit cell. However, while scaling down MOS technology into the deep submicron regime yields area and density benefits to the MRAM bit cells, using these deep submicron devices in the MRAM sense amplifier degrades the read performance (the output swing) of the MRAM sense amplifier due to operating voltage limitations of these deep submicron devices.
Conventional MRAM sense amplifier designs for the 1T1MTJ architecture use core transistors. However, due to breakdown and reliability considerations, the supply voltage for the core transistors is typically limited to about 1 volt (V) for leading-edge deep submicron technologies. The following devices are stacked between the supply rails in conventional MRAM sense amplifier designs for the 1T1MTJ architecture: the bit cell comprising an access transistor and an MTJ device, a mux transistor, a clamp transistor, and a PMOS load transistor. The access and mux transistors should be operated as switches (in the linear region), while the clamp and PMOS load transistors should remain in saturation for the sense amplifier to exhibit reasonable gain. However, the low supply voltage requirement that comes from using core devices in the sense amplifier coupled with the transistor threshold voltage being a significant fraction of this supply voltage can make it difficult to keep the clamp and PMOS load transistors in saturation, which in turn, degrades the gain and results in a sub-optimal signal swing at the sense amplifier output.